The present invention relates to a method for making an electrical connection and to a contact point.
For making an electrical connection between at least two contact points, a single wire contacting, a so-called xe2x80x9cbondingxe2x80x9d is known. In the process, using a wire-bonding device, single wires, in particular gold or aluminum wires are arranged between the contact surfaces like to be contacted. During bonding, the contacting wire is contacted with the contact surfaces under the influence of pressure, ultrasound, and temperature. The connecting wire is initially fused to form a ball at its free end by applying thermal energy, and is then pressed onto the first contact surface using a bonding capillary. In the process, the contacting wire binds to the contact surface by atomic binding forces developing at the interface between the contact surface and the contacting wire. During contacting with the first contact surface, the previously fused ball deforms into a xe2x80x9cnail headxe2x80x9d. Subsequently, the contacting wire is led to the second contact surface using the wire-bonding device. In the process, the contacting wire is led loop to counteract a tearing off of the contacting wire at the first contact surface. The contacting wire is pressed onto the second contact surface using the wire-bonding device, once more under the influence of pressure, ultrasound, and temperature. In the process, the contacting wire is constricted in area so that it forms a predetermined breaking point at which the contacting wire tears subsequent to guiding the wire-bonding device away from the second contact surface. The contacting wire is connected to the second contact surface via a stitch, atomic binding forces developing again at the interface between the contacting wire and the second contact surface.
When working with this known, so-called xe2x80x9cball-wedge bondingxe2x80x9d (ball-contacting with the first contact surface, stitch-contacting with the second contact surface), a strong material dependency exists between the contacting wire and the contact surfaces so that sufficiently great atomic binding forces develop at the interfaces. In particular during contacting the second contact surface, only a relatively small surface connection between the stitch and the contact surface is provided, which, can result in incorrect contactings in particular in the case of contact surfaces made of materials which are hard to bond.
The method according to the present invention has the advantage that the contact stability of the connection between the contacting wire and the second contact surface is significantly improved. By producing a contact metallization composed of the material of the contacting wire on the second contact surface using the wire-bonding device prior to contacting the first contact surface, this contact metallization is available for the later contacting of the contacting wire on the second contact surface. In particular, it is an advantage in this context that contacting can now be carried out between identical materials, namely the material of the contact metallization and of the contacting wire.
It is particularly preferred if the contact metallization is produced by applying a thermal energy to the free end of the contacting wire and subsequently pressing the contacting wire onto the second contact surface under the influence of ultrasonic energy. In this manner, a contact metallization which is used for the later contacting of the contacting wire is prepared at the second contact surface, the contact metallization abutting on the second contact surface over a relatively large surface because of its formation in principle known from the design of the contact point at the first contact surface. In this manner, materials which are hard to bond can also build up relatively great atomic binding forces at the interface between the second contact surface and the contact metallization, the atomic binding forces guaranteeing a high contact stability. The actual contacting of the contacting wire is then carried out on or with the contact metallization, great atomic binding forces being able to act at the interface between the contact metallization and the bonded contacting wire because of the identical materials even in the case of a relatively small contact surface of the contacting wire to be bonded.
It is a further advantage that, by previously producing the contact metallization, the wire-bonding device no longer comes directly into contact with the surface of the contact surface for generating the-necessary pressure during the actual contacting of the contacting wire at the second contact surface. Consequently, this is largely insensitive to topographical influences and substance properties of the contact surface.
Moreover, it is an advantage that, by previously applying the contact metallization for the subsequent contacting of the contacting wire, a metallically clean contact surface which is free of foreign contamination is provided so that an optimum interface connection of the contacting wire is possible.
Furthermore, it is advantageous that, by contacting the contacting wire onto the contact metallization which is composed of the same material, this interface connection can be carried out at room temperature, i.e., without supplying an additional thermal energy during the contacting of the contacting wire since sufficiently great atomic binding forces can be attained already under contact pressure at room temperature. If required, an additional, in principle not necessary supply of a thermal energy can of course take place.
In a further preferred embodiment of the present invention, provision is made for shape features to be stamped onto the contact metallization during its application to the contact surface, the shape features supporting a subsequent contacting of the contacting wire. In this manner, a positive-locking connection between the contacting wire and the contact metallization is advantageously enabled which already allows electrical contacting. A pressure to be applied to the contact point via the wire-bonding device can consequently be limited to the pressure that is sufficient for making the predetermined breaking point of the contacting wire. A higher contact pressure required for attaining an integral connection (atomic binding forces) between the contact metallization and the contacting wire can be dropped, at least be reduced.
Using a contact point, electrical contacting by bonding is advantageously made possible between a contacting wire and a contact surface of different materials of which, due to existing material properties, only low atomic binding forces can be expected at the interface. Because the contact point has a quantity of a contact material, this quantity of the contact material is contacted with the contact surface by an integral connection, the contacting wire is contacted with the quantity of the contact material, and the contacting wire and the contact material are composed of the same material, it is advantageously achieved that, by providing the contact material, a mediation is carried out between the different material properties of the contact surface and of the contacting wire. In this context, in particular because the contact material is applied to the contact surface over a relatively large surface, a good interface connection, i.e., a connection having high contact stability can be carried out even in the case of unfavorable material combinations. Because of the identical material of the contact material and the contacting wire, a later, relatively small contact surface between the contact material and the contacting wire is uncritical for a reliable connection of the contacting wire by bonding.